This invention relates to microelectromechanical systems (MEMS) manufacturing. More particularly, this invention relates to a process for forming moveable features in a composite substrate.
Microelectromechanical systems (MEMS) are generally moveable devices created on a substrate using lithographic techniques common in semiconductor manufacturing. Often, the substrate is a composite substrate including a relatively thick “handle” wafer, a layer of sacrificial material formed or deposited on the handle wafer, and a thinner “device” layer formed or deposited over the sacrificial material in which the moveable feature is defined. After formation of the MEMS device in the device layer, the underlying portion of the sacrificial layer is removed, freeing the device for movement.
For example, a composite “silicon-on-insulator” (SOI) wafer, commonly used in MEMS fabrication, may include a thick silicon handle wafer, about 600 μm thick, over which a sacrificial layer of silicon dioxide (SiO2) is grown to a thickness for example 0.5 to 2 μm. Over the silicon dioxide layer, a device layer of polycrystalline or amorphous silicon may be grown, adhered or deposited. The device layer may generally be thinner than the handle wafer, for example, between about 1 and about 80 μm thick.
Known methods for forming the moveable feature on a composite substrate include forming the moveable feature by, for example, deep reactive ion etching (DRIE). The feature is then released from the underlying oxide layer by etching the oxide layer with, for example, a hydrofluoric (HF) acid etch. Upon removal of the silicon dioxide layer beneath the moveable device, the device is free to move, being tethered to the handle wafer only by the remaining portion of the silicon dioxide layer.
Many MEMS devices are designed to be very sensitive detectors, such that the moveable features involved may have very low stiffness. For example, a MEMS accelerometer may make use of a flexible cantilevered beam, whose displacement under an acceleration is measured. Since the beam may have very low stiffness in order to achieve the desired sensitivity, it may take only a rather small adhesion force to bind the cantilever to the substrate surface, rather than suspended above it, rendering the MEMS device inoperable.